Morphine use and pharmacokinetics in patients with chest pain due to suspected or definite acute myocardial infarction

Abstract

The characteristics of chest pain due to suspected acute myocardial infarction and morphine use during the first 3 hospital days are described in a population of 2988 consecutive patients admitted to hospital. The duration of pain was usually less than 24h (mean 20.9±0.55h), and only 24.8% of patients experienced chest pain of longer duration. The majority of patients had only one attack of pain, but 34.4% experienced four or more attacks during hospitalization. A mean morphine dose of 6.7±0.2mg was administered over the 3 hospitalization days, but surprisingly 52.4% of all patients required no morphine analgesia at all. Independent predictors of an increased morphine consumption were initial degree of suspicion of acute myocardial infarction, ST changes on admission ECG, male sex, a history of angina pectoris and a history of congestive heart failure. In a separate pharmacokinetic/pharmacodynamic study in 10 patients, plasma concentrations of morphine and its major metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), were measured after intravenous administration of morphine. In this patient group, terminal half-life of unchanged morphine ranged from 0.77 to 3.22h. M3G and M6G plasma concentrations increased gradually up to 60–90 min after the intravenous morphine injection. Initial pain intensity by numerical rating scale was 6.6±0.6 (arbitrary units), and after morphine administration, there was a rapid and significant decrease in pain intensity. After 20 min, pain relief was 69±11% and remained at this level during the following 8 h observation period. It is concluded that the need for morphine administration in patients with suspected or definite acute myocardial infarction, differs among subgroups of patients and, in particular, higher doses are needed in those with a strong suspicion of myocardial infarction at arrival. When intravenous morphine is given, it attains full effect 20 min after injection. Furthermore, the active morphine metabolites M3G and M6G appear rapidly in thecirculation, which could influence the analgesic response to morphine treatment.